Recently, active research works have been made on organic EL light emitting devices. Referring to FIG. 2, there is illustrated one typical organic EL light emitting device. As a basic configuration, the device includes a glass substrate 21 and a transparent electrode or anode 22 of tin-doped indium oxide (ITO) or the like formed on the substrate 21. A thin film 23 serving as a hole transporting layer is formed on the anode 22 by evaporating a hole transporting material such as tetraphenyldiamine (TPD). A light emitting layer 24 of a fluorescent material such as an aluminum quinolinol complex (Alq3) is deposited on the layer 23. An electrode or cathode 25' is formed thereon from a metal having a low work function such as magnesium. Such organic EL devices are attractive in that they can achieve a very high luminance ranging from 100 to 1,000 cd/m.sup.2 with a drive voltage of approximately 10 volts.
The cathode of such organic EL devices is generally formed of a material capable of injecting more electrons into the light emitting layer. Differently stated, a material having a lower work function is believed suitable as the cathode. There are known many materials having a low work function. The materials which are known effective as the cathode of EL light emitting devices include alloys such as MgAg and MgIn as described in JP-A 15595/1990 and combinations of an alkali metal and a metal having a high work function, for example, intermetallic compounds such as AlCa and AlLi.
Where the organic EL device is constructed to the structure shown in FIG. 2 wherein the cathode 25' is not light transmissive, light emission is taken out from the anode 22 side. This limits the manner of application of the organic EL device when it is used as a light emitting device or display device.
When the organic EL device is used as a display device such as a matrix display, a part of light emission is reflected by the cathode thin film 25' and the reflected light also comes out from the anode 22 side. In the event of the cathode 25' having a particular reflectivity, the reflected light is diverted to reduce the contrast of the display screen. On outdoor use or use in a brightly illuminated place, sunlight or external intense light enters the device to produce intense reflected light which causes to further reduce the contrast.
One known attempt to take out light emission of an organic EL device from both the cathode and anode sides is the use of Mg-Ag and ITO in the cathode as described in SID 96 DIGEST.185 14.2: Novel Transparent Organic Electro-luminescent Devices, G. Gu, V. B. Bulovic, P. E. Burrows, S. R. Forrest, M. E. Tompson. In this device, the cathode is constructed by forming Mg-Ag on the light emitting layer of Alq3 to a thickness of 100 .ANG. and then depositing ITO thereon. The device produces light emission at a luminance of 500 cd/m.sup.2 with a drive voltage of 10 volts while both the cathode and anode provide substantially equal light transmission in the wavelength range of 480 to 570 nm.
The cathode consisting of Mg-Ag and ITO layers, however, has low efficiency since the ITO layer as deposited does not have a low resistance. In order that ITO have a low resistance at room temperature, it must be heat treated. The heat treatment can damage the organic EL device to impair its performance.